Assessment of Antimicrobial Exposure on Generic <i>Escherichia coli</i> and <i>Enterococcus</i> spp. Concentration, Prevalence, and Resistance to Antimicrobials in Beef Cattle Raised with or Without Antimicrobials

The aim was to longitudinally evaluate the association between antimicrobial exposure and resistance occurrence within generic <i>Escherichia coli</i> and <i>Enterococcus</i> spp. populations in feedlot beef cattle raised conventionally or raised without antimicrobials. Angus steers (<i>n</i> = 180) were sorted into 1 of 2 treatments over 2 consecutive years (108 in yr 1 and 72 in yr 2): steers raised without antimicrobials (NAT) and conventionally raised steers exposed to antimicrobials (CONV). Pens within treatment were adjacent and separated by five empty pens from the other treatment. Monensin and tylosin were included in CONV steer diets. On d 123, CONV steers received a metaphylactic antimicrobial. Longitudinal diet (<i>n</i> = 6/year) and fecal (<i>n</i> = 5/year) sampling timepoints were collected to determine <i>E. coli</i> and <i>Enterococcus</i> spp. concentration, prevalence, and resistance patterns. Dietary <i>Enterococcus</i> spp. concentrations, and erythromycin (8ERY^R; 128ERY^R), tetracycline (TET^R), trimethoprim-sulfamethoxazole (COT^R), and cefotaxime (CTX^R) resistant <i>E. coli</i> concentrations and prevalence were greater in NAT diets than CONV diets (<i>p</i> < 0.02). Fecal <i>E. coli</i> concentrations tended to be greater in NAT steers than CON steers (<i>p</i> = 0.07). Fecal TET^R <i>E. coli</i> concentrations were greater in CONV steers than NAT steers (<i>p</i> = 0.03). Fecal COT^R and CTX^R <i>E. coli</i> prevalence was greater for CONV steers at the beginning of the finishing phase while greater for NAT steers at the end of the finishing phase (<i>p</i> < 0.01). Fecal <i>Enterococcus</i> spp. concentrations did not differ between treatments (<i>p</i> = 0.11). Concentrations of 8ERY^R and 128ERY^R <i>Enterococcus</i> spp. were greater in CONV steers on d 64, 130, and 168 than NAT steers (<i>p</i> < 0.05). Overall, antimicrobial resistant <i>Enterococcus</i> spp. and <i>E. coli</i> were detected regardless of antimicrobial exposure.